In the treatment of infants with respiratory disease it can be of great importance to measure the concentration of carbon dioxide and oxygen in the expelled air of the infant. It is even more useful if these gases, particularly the carbon dioxide, can be measured at the end of expiration so as to obtain the highest carbon dioxide concentration of the expiration, usually called end-tidal peak carbon dioxide (ECO.sub.2). Likewise, in the study of infants liable to "Sudden Infant Death Syndrome", the measurement of end-tidal peak carbon dioxide can be of critical importance because such infants may hypoventilate with a rising ECO.sub.2 before going into apnea and dying.
In adults, who breathe at a rate of about 20 breaths per minute, it is relatively easy to take measurements of ECO.sub.2. But most current rapid analyzers are limited in their response time to indicating only about 90% of the actual concentration in two tenths of a second. This means that in infants, who breath at a high rate, often over sixty or eighty, the response of a rapid gas analyzer is too slow to pick the peak of the end-tidal carbon dioxide because the next inspiration intervenes before the instrument has a chance to come to equilibrium for a valid measurement. Reliable measurements of end-tidal carbon dioxide in infants have heretofore only been possible using specially adapted mass-spectrometers which are very expensive and even then the end-tidal peak is often blurred by the rapid respiration rate.
Another problem in such measurement with respect to infants is that in order to measure, it is necessary to draw a continuous sample from the airway, typically at the rate of about 0.5 liters per minute and pass that volume through the gas analyzer. But in an infant who is only breathing one or two liters per minute and who is on the partially closed circuit of a ventilator, the half liter is a dangerously large amount to suck out of the airway.